Surface tension means a pulling force of liquids on a surface and has a feature of minimizing the surface by means of intermolecular forces. Liquid molecules apply force to each other. A single liquid molecule receives forces from surrounding molecules in all direction. However, on a surface, there is no force applied from the above. Therefore, a state on a surface is more unstable in comparison to an intermediate location, and in order to minimize such an unstable state, liquid tends to minimize the surface which is in an unstable state.
Such instability may be reduced by applying a force to another particle on the surface. In other words, a balance of intermolecular attractions of a liquid is broken, molecules near the liquid surface have greater potential energy in comparison to molecules inside the liquid, and for this reason, the liquid has energy proportional to the entire surface energy, thereby creating surface tension. Therefore, the surface tension may also be expressed as an equation in relation to the energy applied to a unit area. Actually, the surface tension is expressed by a tension applied to both sides of a line having a unit length. This value is a constant determined according to the kind of liquid but may also vary depending on temperature.
As described above, the surface tension is in form of attractions applied among all molecules located on a surface, and this keeps a balance with a resisting force against compression of the liquid. Therefore, in a physical aspect, the surface tension is applied in a tangential direction of the liquid surface, resultantly forming a curvature of a swelled balloon or bubble.
Generally, the force of surface tension includes a Laplace formula which expresses a pressure jump on a boundary.
If surfaces of two different materials come into contact with each other, a contact angle is formed. At a contact surface with a solid wall, the wetting effect may be expressed using the contact angle. The contact angle represents an angle formed when liquid keeps a thermodynamic equilibrium on a solid surface. At this time, the contact angle varies according to a feature of the fluid or a roughness or shape of the wall.
Such a contact angle is generated by different surface free energy between liquid and solid, between liquid and gas, or between gas and solid. If three different materials come into contact with each other, such a contact angle is generated, and equilibrium is kept at a certain angle by means of each surface free energy. Generally, the contact angle is used as an angle formed by a solid surface and a liquid surface.
However, if fluid moves, such a contact angle changes. This is called a dynamic contact angle. The dynamic contact angle represents an angle formed between solid and fluid surfaces when the fluid flows, and is frequently used when measuring surface tension of a solution. The dynamic contact angle is generally generated by inertia in relation to the movement of fluid.
In order to measure surface tension of a liquid, various studies have been performed.
(1) Capillary rise method. A capillary tube is placed upright in a specimen solution, and a height (h) of the specimen solution rising in the capillary tube is measured. Here, surface tension=rhρg/2, where r represents a diameter of the capillary tube, ρ represents a density of the specimen solution, and g represents an acceleration of gravity.
(2) Stalagmometric method. A stalagmometer (a circular tube having a flat section) is used to quietly drop a specimen solution. Assuming that a single drop has a mass of m, a force of pulling downwards a dropping liquid is mg, and a force of lifting a dropping liquid upwards is 2πγ×(surface tension). γ represents an outer radius of the tube, and the surface tension is calculated by measuring m.
(3) Pendent drop method. A small pellet of platinum is suspended horizontally and brought into contact with a specimen solution, and then when pulling the small pellet of platinum, a force f required for detaching the pellet of a radius γ from a liquid is a double of 2πγ. Surface tension is calculated from f by using a torsion balance. A surface tension measurer of Du Nouy uses this method and is available in the market. In this method, surface tension of a small amount of specimen solution may be measured in a relatively rapid and accurate way.
(4) Stalagmometric comparison method. A stalagmometer is used to drop a certain amount of specimen solution, and the dropped amount (n2) is checked. The same volume of standard solution is dropped using the same stalagmometer, and its dropped amount (n1) is calculated. Assuming that the density of the specimen solution is d2 and the density of the standard solution is d1, surface tension of the specimen solution=surface tension of the standard solution×(n1/n2)×(d2/d1).
FIG. 1 is a representative diagram showing a method for measuring a magnitude of surface tension by using an existing optical method.
The surface tension may be measured according to Equations 1 and 2 below. Here, Ci represents a curvature of a medium i.
                              ρ          ⁢                                          ⁢          g          ⁢                                          ⁢          Δ          ⁢                                          ⁢          h                =                  γ          ⁡                      (                                          C                1                            -                              C                2                                      )                                              (                  Equation          ⁢                                          ⁢          1                )                                γ        =                              ρ            ⁢                                                  ⁢            g            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            h                                (                                          C                1                            -                              C                2                                      )                                              (                  Equation          ⁢                                          ⁢          2                )            
When measuring surface tension in this way, an optical method is mostly used, which however has complicated measurement and analysis processes and causes an error.
In the present disclosure, a new method for measuring surface tension is proposed using an electro-wetting property. Electro-wetting means a phenomenon in which when a voltage is applied between a conductive fluid and a bottom electrode as shown in FIG. 9, wettability to the bottom surface of the conductive fluid changes and the conductive fluid spreads on the bottom surface.